Known power steering systems include a power steering pump, a valve actuated upon turning of the vehicle steering wheel, and a power steering motor for moving the vehicle wheels in response to actuation of the valve. The pump is driven from the engine. When the engine is operating at a relatively low speed (revolutions per minute), the pump must deliver sufficient fluid at the pressure created by the steering load to power the steering motor and steer the vehicle wheels.
A typical truck steering pump delivers 3.6 gallons per minute at an engine speed of 600 rpm. At an engine speed of 3000 rpm, the same pump would deliver 18 gallons per minute. Such a fluid flow rate would create a high pressure drop across the steering valve and consume horsepower. To avoid this problem, for years pumps used in power steering systems have been provided with flow control valves which operate to bypass excess fluid flow delivered from the pump directly to tank.
Known flow control valves are controlled by fluid pressure forces. Fluid from the pump flows through a flow control orifice to the power steering valve. The pressure drop across the orifice is communicated to the flow control valve. As pump outlet flow increases and more flow passes through the orifice, the pressure drop across the flow control valve increases. When the force created by the pressure drop across the valve exceeds the spring force holding the valve closed, the valve will open and bypass flow to return. When pump flow decreases due to increased system pressure, the pressure drop across the flow control valve decreases and the flow control valve closes to bypass less fluid to return. In this way, the flow control valve tends to maintain a constant pressure drop across the flow control orifice and thus constant flow to the system. There are many patents relating to flow control valves and their controls. U.S. Pat. Nos. 3,384,020 and 4,513,835 are typical examples of such patents.
A power steering system is described in SAE Technical Paper Series No. 841198 dated Oct. 1, 1984 entitled "Development of an Electronically-Controlled Power Steering System." The described system includes a flow control valve for bypassing flow directly from the power steering pump to tank. The flow control valve includes a solenoid actuated valve member. The valve member is moved to control the amount of fluid bypassed according to vehicle speed.
In the power steering system described in the SAE Paper No. 841198, the flow control valve must be a relatively large valve to bypass substantial amounts of hydraulic fluid. Therefore, a relatively large solenoid must be used to control the valve member. A large solenoid requires relatively high operating current. Solenoid operated valves having a large solenoid typically have a slow response time and an undesirably large operating hysteresis. Also, such large solenoid valves are relatively expensive.
Some power assist steering systems control power assist in response to vehicle speed. During operation of a vehicle at a relatively low speed, there is a relatively large resistance to turning movement of the vehicle wheels. It is desirable to provide maximum power assist for such low speed maneuvering. As vehicle speed increases, resistence to turning movement of the vehicle wheels decreases. Therefore, as the vehicle speed increases, less power assist is needed to provide an acceptable steering "feel" for the operator. One power assist system has been developed to bypass fluid directly from a pump to a resevoir wherein the amount of fluid bypassed is controlled in response to the vehicle speed. This power assist steering system is disclosed in co-pending U.S. patent application Ser. No. 738,601, and is assigned to the assignee of the subject application.